This invention relates to a signal recovery systems and more particularly to a recovery system for a vestigial sideband (VSB) digital signal.
Generally, the United States Grand Alliance (GA) utilizes the VSB transmission method as the broadcasting standard for a high definition television (HDTV). The standardized VSB transmission signal includes a specific synchronization (sync) signal pattern within the data stream which allows a HDTV receiver to easily recover the sync signal. The data stream incorporates consecutive data segments, wherein each segment consists of 832 symbol units and within the 832 symbol units, four symbols such as +1, 0, 0, +1 define a sync signal pattern while the other 828 symbols are data information. To recover the segment synchronization signal, the specific four symbol pattern must be found.
FIG. 1 shows a general block diagram of a HDTV receiver system including an analog signal processor 12 for receiving the transmission signal through the antenna 10, a low pass filter (LPF) 14 for low pass filtering the signal received from the analog signal processor 12, an analog/digital converter (ADC) 16 for converting the filtered analog signal to a digital signal, and a data Segment (DS) sync detector 18 for detecting and recovering the segment sync signal pattern from the digital signal received from the ADC 16. The recovered segment sync signal is output to the equalizer.
FIGS. 2, 3 and 4 show the effects of ghost signals in the 5 output waveforms of a HDTV receiver system described above in reference to FIG. 1. Particularly, FIG. 2 shows the output waveforms of a HDTV receiver system wherein the transmitted signal is undistorted by ghost signals. FIG. 2(a) shows the system clock, 2(b) shows the waveform of the low pass filtered signal output by LPF 14, and 2(c) shows the waveform of the digital sync signal after the analog signal was converted by the ADC 16. The DS sync detector 18 determines whether the digital signal is a sync pattern by comparing the digital signal pattern to a predetermined sync pattern.
FIG. 3(a) shows the system clock and 3(b) shows the waveform of a digital pattern which must be detected by the DS sync detector 18. FIGS. 3(c)-3(f) shows the low pass filtered waveform of each data segment sync symbols 1, 0, 0, 1 individually and 3(g) shows the combination of the waveforms shown in 3(a)-3(f). As shown in 3(g), the amplitude of sync symbol 0 is greater than the amplitude of sync symbol 1 in the low pass filtered waveform output by the LPF 14. The greater amplitude results from the addition of two consecutive 0 waveforms as shown in 3(d) and 3(e).
FIG. 4 shows the effects on the waveforms when ghost signal distorts the first 1.5 sync symbol portion. FIG. 4(a) shows the system clock, 4(b) shows the addition of ghost signal to the original data segment sync pattern, 4(c) shows the output waveform of the original and ghost signal as the signals pass through the LPF 14, and 4(d) shows the combination of the original sync waveform and the ghost waveform after the signals have passed through the LPF 14. Note in 4(c), the xcex3 ghost signal portion corresponds to the xcex1 sync signal portion of the original sync pattern. Also, if the amplitude of xcex3 portion is greater than xcex2 portion, the combination of the waveforms falls below the baseline as shown by xcex8 in 4(d). Accordingly, if the baseline serves as the threshold for slicing the analog signal to a digital signal, the DS sync detector 18 cannot recognize xcex8 portion as a xe2x80x981xe2x80x99. Therefore, if the waveform shown in 4(d) is converted to a digital signal by it the ADC 16 according to the system clock, the DS sync detector 18 cannot detect the data segment sync pattern of 1, 0, 0, 1 because the original pattern was distorted by ghost signal.
An object of the present invention is to solve at least the problems and disadvantages of the related art.
Another object of the present invention is to provide a more efficient sync signal detector for a HDTV receiver.
A further object to the present invention is to eliminate the distortions caused by ghost signals.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purposes of the invention, as embodied and broadly described herein, the present HDTV receiver includes a pre-filtering processor before the signal is output to the sync detector to detect and recover the sync signal.